As is well known, a filter is placed in the exhaust system of diesel engines to remove soot from the exhaust gases of the engine. The filter must be changed or cleaned from time to time to ensure that soot accumulations do not adversely affect the operation of the engine. It is known to remove or incinerate the soot particles by subjecting the filter, in situ, to heat from a fuel burner or other heat generating device, or from suitable running of the engine. Incineration is to be performed when the accumulation has reached a level where further accumulation would adversely affect engine performance or that incineration would produce excessive temperatures and possibly damage the filter. There is a need, therefore, for a method and apparatus which monitors the level of soot accumulation and provides a signal when the accumulation reaches a predetermined level.
It is also known that soot accumulations exhibit dielectric properties. Accordingly, it is possible to monitor the level of soot accumulation on a diesel engine filter medium by detecting changes in the effective dielectric properties of the filter medium. By way of background, the complex permittivity of a material is comprised of two components: a real component called the dielectric constant and an imaginary component called the dielectric loss factor. Changes in either of these components can be detected using microwave interrogation methods.
One method of detecting changes in the effective dielectric constant involves exciting a microwave waveguide or transmission line, in which the filter is housed, with microwave energy at a fixed frequency and measuring the reflected power. For any RF system, a frequency can usually be found such that the electrical load, i.e. the filter medium, the diesel soot and the filter containment in this case, represents a matched impedance with respect to the power source. In other words, the equivalent electrical resistance, capacitance and/or inductance of the load are matched to the RF power source. When the load impedance is perfectly matched to the power source, all emitted RF power is absorbed by the load. If the impedance is not matched to the RF source, some of the RF power will be reflected from the load. The degree of load mismatch determines the amount of reflected power and hence reflected power can be used to measure the change in the effective dielectric constant.
U.S. Pat. No. 4,477,771 granted to the General Motors Corporation on Oct. 16, 1984 describes a method of detecting soot content in a particulate trap using this method. More specifically, the method is based on the principle of detecting changes in the effective dielectric constant only. The patent provides a filter housing which forms a closed, microwave resonance cavity in which a ceramic filter is placed. A single probe is positioned at one end of the cavity and behaves as both a transmitting and receiving antenna. A reflective screen is positioned at the opposite end of the cavity. All connecting exhaust pipe diameters are below the cutoff diameter of a circular waveguide needed to transmit the RF energy at the frequencies used in the device. The probe is connected to a microwave source through a directional coupler and an isolator. A detector is also connected to the probe through the directional coupler. In one mode of operation of the device, the microwave source is operated at the resonant frequency of the cavity when the filter is loaded with particulates to its maximum desired accumulation and the detector is operated to detect a null condition in the reflected signal which occurs at the resonant condition. Upon detecting such a condition, the detector generates an output signal operable to effect operation of a lamp or alarm. In a second embodiment, the reflective screen is eliminated and a second probe is positioned at the remote end of the cavity. One probe is connected to the power source and the other probe is connected to the detector.
It has been found that there are a number of technical and practical problems with this approach. First, the isolators and directional couplers are relatively complex devices needed to protect the RF source from the reflected power. From a practical point of view, the cost of these items would almost certainly preclude the device from being commercialized. Second, a power source of sufficient stability to allow long-term measurements at a single frequency without frequency drift would be prohibitively expensive. Third, and perhaps most importantly, the device tends to display poor sensitivity and is prone to large measurement errors due to the effect of temperature on the effective dielectric constant.
Another method involves exciting a microwave waveguide or transmission line with microwave energy at variable frequencies and measuring the reflected power. This is simply an extension of the method described in the General Motors patent. In accordance with this approach, the frequency is varied in order to minimize the reflected power. In effect, the frequency is varied in order to match the RF power source characteristics to the load impedance characteristics. Structurally, the device is the same as that described above except a variable frequency source is required. Frequency is used as the measurement parameter instead of reflected power. In addition the drawbacks discussed earlier, a variable frequency source and the required control logic would make this type of device prohibitively expensive.